Hydrogen sorption and reaction mechanisms of nanoconfined 2LiBH4–NaAlH4

Abstract Nanoconfinement of 2LiBH 4 –NaAlH 4 in carbon aerogel scaffold (CAS) via direct melt infiltration is proposed for reversible hydrogen storage. Weight ratios of CAS:2LiBH 4 –NaAlH 4 (2:1, 2:1.5, and 1:1) and melt infiltration temperatures at 185 and 310 °C are varied to obtain the most suitable condition for nanoconfinement. It is found that the sample melt infiltrated at 310 °C under CAS:2LiBH 4 –NaAlH 4 weight ratio of 2:1, denoted as s1@310 offers the best results based on effective nanoconfinement, high hydrogen content released, and fast dehydrogenation kinetics. Further studies on reversibility shows that up to 89% and 74% H 2 with respect to theoretical storage capacity release from s1@310 during the 1st and 2nd dehydrogenations ( T  = 400 °C under vacuum), respectively, while those of milled 2LiBH 4 –NaAlH 4 are only 57% and 32%, respectively. The reaction between LiBH 4 and NaAlH 4 during nanoconfinement produces LiAlH 4 and NaBH 4 . During dehydrogenation, an intermediate phase (LiNa 2 AlH 6 ) is formed and it decomposes in the temperature range of 203–350 °C. Moreover, significant reduction in decomposition temperature of NaBH 4 of s1@310 with respect to those of milled 2LiBH 4 –NaAlH 4 and pristine NaBH 4 (Δ T  = 137 and 197 °C, respectively) is accomplished. Reversibility of NaBH 4 , LiAlH 4 , and LiBH 4 are achieved together with unreacted LiH and metallic Al ( T  = 400 °C, p (H 2 ) = 80 bar for 12 h). Although irreversibility of metallic Al leads to deficient content of hydrogen desorbed in the 2nd cycle, greater amount of hydrogen reproduced of s1@310 with respect to milled sample is obtained. This is due to the recovery of NaBH 4 after rehydrogenation of s1@310, whereas that of milled 2LiBH 4 –NaAlH 4 was not possible due to the evaporation of metallic Na during dehydrogenation.